1. Field of the Invention
This invention relates to a process for preparing a vinyl chloride polymer through a suspension polymerization in an aqueous medium.
2. Description of the Prior Art
Conventionally, it is known to add an antioxidant to a polymerization system when preparing a vinyl chloride polymer, which is added for the purposes of (1) retarding or terminating the polymerization reaction, (2) homogeneously dispersing the antioxidant in a resulting vinyl chloride resin product to prevent the vinyl chloride resin from being deteriorated, etc. In any instances, the antioxidant is required to be homogeneously dispersed in the suspension polymerization system in order to bring the effect by the addition of the antioxidant into the maximum.
The antioxidant conventionally includes phenol antioxidants, amine antioxidants, sulfur antioxidants, phosphorous antioxidants, etc., which, however, are powdery in general, making it difficult to charge them in an accurate desired amount by way of a mechanical means such as pumps. In particular, the addition of the antioxidant during polymerization needs be carried out by pressing it into the polymerization which is kept in a pressurized state. It is difficult to press the powdery antioxidant in an accurate desired amount into the system, and it is also difficult to adjust the amount of the antioxidant to a predetermined amount.
For instance, U.S. Pat. No. 4,482,684 describes that the polymerization inhibitor disclosed therein can conveniently be incorporated into the polymerization reaction mixture in solid form at the beginning of the reaction (See column 3, lines 9-12), but this patent describes no method of charging the polymerization inhibitor in a polymerization system in the pressurized state. Moreover, the charging of the antioxidant in the said powdery state generally results in poor dispersion, thereby causing the drawbacks that the vinyl chloride polymer obtained exhibits increase in fish eyes and lowering of quality.
For this reason, it has been conventionally practiced to dissolve the antioxidant in a suitable organic solvent and add it as a solution. This method makes it possible to charge the antioxidant in the polymerization vessel by way of a mechanical means such as pumps, but has had the problems such that it becomes necessary to recover the solvent, the solvent remains in the resulting vinyl chloride resin to cause a poorness of the quality, the solvent mixes into the unreacted monomers to be recovered, and the COD in waste water increases in the instance of the suspension polymerization carried out in an aqueous medium.
As a method of charging the antioxidant into a polymerization kept in a pressurized state without using any organic solvent, U.S. Pat. No. 4,619,978 describes a method in which a mixture of an antioxidant and vinyl chloride monomer is heated to produce a pressure equal to the pressure in the polymerization vessel and thereafter charged into the vessel (See column 7, lines 1-14). This method has the disadvantage that the charging procedure is complicated. This method also has the drawback that if the antioxidant is charged at the end of polymerization, removal of unreacted monomers takes longer time after the polymerization.
Now, there has been recently proposed a process in which a powdery antioxidant is dispersed in water by using an emulsifying agent alone or using the emulsifying agent in combination with a suspending agent, to bring it into a state of an emulsion, which is then added in the polymerization system (Japanese Unexamined Pat. Publications No. 149608/1985 and No. 186547/1985).
The above process in which the antioxidant is added in the form of an emulsion by use of an emulsifying agent may lower the thermal stability and storage stability of the resulting vinyl chloride resin because of the inclusion of the emulsifying agent therein. It also has the problems such that, when added for the purpose of terminating the polymerization, the reaction tends to be terminated with insufficient effect, and, when added at the time the polymerization is completed, it may follow that the plasticizer absorption performance of the vinyl chloride resin becomes poor after the resin is subjected to heat treatment.